Development of a secondary battery having high energy density and free from environmental pollution has lately been required.
A conventional manufacturing method of a lithium secondary battery is explained below in conjunction with FIGS. 8 to 10. Since the steps to manufacture a negative electrode and a positive electrode are the same, only the steps to manufacture the positive electrode are explained below.
In FIG. 10, a positive electrode (40) and a negative electrode (50) are wound as spiral coils with a separator (60) interposed therebetween, and are housed in a battery can (61). The positive electrode (40) is connected to a positive electrode external terminal (42) through a positive electrode tab (41). The negative electrode (50) is also connected to a negative electrode external terminal (52) through a negative electrode tab (51). An insulating packing (65) comprising a pair of insulators (63) and (64) is inserted at connected areas of a sealing board (62) and the positive electrode external terminal (42) and a sealing board (62) and the negative electrode external terminal (52). The insulating packing (65) is fastened by a pair of nuts (66) from both sides, i.e. upper and bottom, and is fixed to make the lithium secondary battery airtight.
FIG. 8 illustrates a positive electrode tab (41) joined to a positive electrode (40) in a conventional lithium secondary battery. The positive electrode (40) includes a current collector (48) having coated portions on both surfaces of active material layers (47) comprising an active material, an electrically conductive filler, and the like, and uncoated portions (49) provided at regular intervals. A positive electrode tab (41) is joined to an uncoated position on the surface of the current collector (48) at an end portion of the positive electrode tab (41).
A method of joining positive electrode tab (41) to positive electrode (40) is concretely explained below in conjunction with FIG. 9. The uncoated portion (49) and the positive electrode tab (41) are placed together as shown in FIG. 9(a), and are joined to each other by spot-welding. At this stage, the current collector (48) and the positive electrode tab (41) are joined only at portions where electricity passed during spot-welding, and remaining portions are not joined as shown in FIG. 9(b). Pressure is applied to the current collector (48) and the positive electrode tab (41) at the joint area vertically by a pressing machine to increase the contact area and to reduce contact resistance between the current collector and the positive electrode tab.
A conventional current collector and electrode tab are formed by rolling, and the surface of the electrode tab is a so-called rolled finished surface (44). Portions of the electrode tab which are not joined by spot-welding become separated, or detached, after the pressing process by so-called elastic reset as shown in FIG. 9(c). This causes a reduction in the contact area, increases internal resistance of the battery which is caused by contact resistance, and reduces output at high current discharge and reduces discharge capacity of the battery.
It has been disclosed that a contact area of a electrode active material and a current collector can be increased to reduce contact resistance and to improve output property (Japanese Patent Laid-open Hei 8-195202). However, contact resistance between an electrode tab and a current collector has not been reduced.